High strength barium titanate ceramic bodies

ABSTRACT

THE MECHANICAL STRENGTH AND DENSIFICATION OF BARIUM TITANATE CERAMIC BODIES ARE ENCHANCED BY THE ADDITION OF SMALL AMOUNTS OF HALIDE SALTS IN COMBINATION WITH METAL OXIDES. TRANSPARENCY OF BARIUM TITANATE BODIES ARE ALSO ATTAINED BY THE ADDITION OF AN ALKALI FLUORIDE.

United States Patent Otfice 3,753,911 Patented Aug. 21, 1973 U.S. Cl. 252--62.9 Claims ABSTRACT OF THE DISCLOSURE The mechanical strength and densification of barium titanate ceramic bodies are enhanced by the addition of small amounts of halide salts in combination with metal oxides. Transparency of barium titanate bodies are also attained by the addition of an alkali fluoride.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to ceramic materials and, more particularly, to barium titanate ceramics having improved strength and density properties.

Description of the prior art roelectric and dielectric characteristics. The combination of at least about 0.1% halide salts selected from the group consisting of alkali metal fluorides and LiCl with at least about 0.5% MgO or La O and the balance BaTiO comprise the ingredients for the high strength ceramic bodies. Standard manufacturing techniques such as, for example, cold pressing, sintering and annealing or hot pressing plus annealing are suitable to produce the novel ceramic bodies for use as piezoelectric elements, capacitors, and electrooptical elements.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the present invention an improved piezoelectric ceramic body, based on barium titanate, can be made with standard ceramic techniques having unexpectedly high mechanical strength if small amounts of a halide salt and a metal oxide are added to the ceramic mixture. By the phrase standard ceramic techniques it is meant that the barium titanate mixes may be cold pressed and then fired under various controlled environmental conditions and annealed or hot pressed with subsequent annealing.

Properties of barium titanate powders useful with the present invention are listed and compared in Table I. These powders, which are available commercially, were pressed and annealed into ceramic bodies and subsequently tested for mechanical strength on an Instron test machine. Density was determined by the buoyancy method and the approximate grain size and porosity were determined with optical and electron microscopy techniques. The above testing methods were also utilized on the ceramic bodies set forth in Tables II-V. As can be seen in Table I, Sample A had a 1.5% impurity level but also had the greatest strength. (This is likely the result of its small grain size.) The impurities present in the powders were primarily hydroxides and carbonates, however, their presence was of no effect since they substantially all disappeared during heat treatment of the test specimens.

TABLE I Wt. percent Percent Modulus of Average BaTiOa mpur- Addi- Annealing theoretical rupture particle sample powders 1 ities tives temp, C. density (R), p.s.i. size, [.L

1 Samples were hot pressed 10-50 min. at LOGO-5,000 p.s.i.; at 9001,200 0., and annealed for about 20 hours.

strength, cause manufacturing difficulties and lack of uniformity from batch to batch due to their high vapor pressure SUMMARY OF THE INVENTION The present invention provides novel barium titanatebased ceramic bodies having improved densification and mechanical strength properties without impairment of fer- Table II illustrates the results obtainable when various single ingredients are added to the barium titanate mixes. Commercially manufactured barium titanate transducer rings containing calcium titanate as an additive were also tested and listed for comparison purposes. Table III is similar in that it shows the mechanical strength properties of multiple ingredients in combination with barium titanate.

TABLE II Wt. percent An- Percent Modulus of neahng theorutpure Average BaTiO; Impurtemp., retical (R), particle sample powders 1 ities Additives 0. density p.s.i. size, p

f1 2-4. 0 5% Ca'IiO; 16,000 #2 2-4. 0 5% CaTiO; 94 15, 000 #3 2 2-4. 0 5% CaTiOa 95 14, 000 A. 2% LiF 98 15, 000 A 2% MgO 95 7, 000 A 1% MgF-z 95. 5 9, 000 A--- 2% Zn 99 17,000 B 2% Lil 95. 5 8, 500 D 2% LlF 97. 5 14, 500

1 Samples were hot pressed l050 min. at LOGO-5,000 p.s.i.; at 9001,200 O. and annealed for about 20 hours.

2 Commercial Ba'liOa Transducer Ring Test samples.

TABLE III An- Percent BaTiOs nealing theo- Modulus of Average sample Wt. percent temp., retical rupture particle powders additives density (R),p.s.i. size, n

A ugpggfi gg 1, 400 02. 0, 000 3 A {f:g;; R5 1,000 as. o 17, 000 3 A--. "6 8;; 2 1,100 91. 0 1a, 000 e A 55 1,000 00.0 10,000 1 A- 3 2g; 1, s00 95. 0 14, 500 1 Table IV exemplifies the present invention and demonstrates the unexpected improvement in mechanical strength of hot pressed barium titanate ceramics by the addition of varying amounts of alkali metal fluorides, magnesium fluoride and lithium chloride in combination with magnesium oxide. As shown, at least 0.1% of a halide salt in combination with magnesium oxide significantly improves the mechanical strength of the barium titanate-based ceramic bodies. La o can be used in place of the MgO, and other alkali fluorides such as, for example, potassium fluoride, are suitable for use in place of the fluorides listed in Table IV. Additionally, although only two additives are shown in combination with the BaTiO' various combinations of three or more of the additives can be utilized. The samples were hot pressed at 1000 C. and annealed for 20 hours at the temperature indicated. The optimum annealing temperature was between 1000 and 1200 C.

such as a vacuum or hydrogen atomsphere, prior to annealing.

The dielectric properties of various barium titanatebased ceramics of the present invention are listed in Table V and compared to typical commercial samples. As shown, the loss factors and dielectric constants of the high strength materials compare favorably with the prior art. In addition to being useful as dielectric bodies the ceramics of the present invention show transparencies for radiation in the visible region for samples up to 0.2. inch thick. Tests using a 2854" K. tungsten filament light source show inline, i.e., normal, light transmission values on 0.010 inch samples of 2 to 5% relative to a single crystal BaTiO Samples without the additives of the present invention gave less than 1% normal light transmission. The presence of LiF in the ceramic bodies appeared to be a major factor in providing the transparency. As a result, the present invention has importance for use as electro-optical switching elements. Additionally, piezoelectric tests on typical barium titanate samples of the present invention containing LiF-MgO or NaF-MgO showed apparent coupling factors of about 0.1. This is a significant response since commercial samples range from 0.1 to 0.2.

What is claimed and desired to be secured by Letters Patent of the United States is:

1. A BaTio piezoelectric ceramic body consisting of:

at least about 0.1% by weight of a halide salt selected from the group consisting of alkali metal fluorides, MgF and LiCl;

at least about 0.5%, by weight MgO or La 0' and the balance BaTiO TABLE IV Anneal- Percent ing theo- Modulus Average BaTiOs sample Wt. percent temp, retical of rupture grain powders additives 0 density (R), p.s.i. size, n

.0. if; as" A. 0.5 MB plus MgO 1:100 8 3g 2g 1' tit 32% 28088 P3 LiF P Mgo Z200 0 2 000 0 96. 5 21, 000 1. 0 A; 1.0 MgFz plus 1.0 MgO.--. 1, 000 97. 0 23, 000 5, 0 an a" .0 C HF Plus Mgo 1,100 00.0 19, 000 1.0 l, 000 98.5 23,000 3.0 A 1.0 NaF plus 1.0 MgO 1, 200 99.0 20, 000 10,0 1, 300 98. 5 23, 000 5. 0 1.0 L1]? plus 2 MgO.....--- 1,000 96.0 25,000 1.0 02 Lil? plus 2.0 MgO 200 99.0 27, 000 1 0 A 1.0 LiF plus 112.203 1, 150 98.5 000 1 0 1 Room temperature.

TABLE V Anneal- Percent Average ing theomodulus Average Loss temp, retical of rupture grain Dielectric factor, 0 density (R), psi. size, )4 constant percent Wt. ercent additives:

( .5 LiF plus 9 0 MgO. 1, 200 99.0 29, 500 150 3, 300 0. 3 1,000 98.0 24, 500 10 2,300 0.5 1, 000 98. 5 23, 000 3 11, 100 0.8 1, 200 99. 0 20, 000 10 4, 200 0. 0 1, 300 98. 5 23, 000 5 7, 700 0. 5 1.0 MgF plus 1.0 MgO 1, 000 97- 0 000 5 3, 300 0. 3 0.5 LiF1plus 1.10 MgO plus Ta20a---- 1, 000 98. 0 18, 500 5 4,450 0.3

a sum 0: iii p 05.0 16,000 8 2,000 1.0 #2 94 15, 000 2 2, 000 1. o #2 95 14, 000 2 2,000 1.0

Other prior art additives may be used with the halide said ceramic body exhibiting a higher average modulus salt-metal oxide combination to effect specific purposes. of rupture h a BaTiO ceramic b d i h id Illustrative of such additives 1s magnes1um t1tanate, cohalide salt and g or bah manate caicmm manate .alumuium made tamahim 2. The ceramic body of claim 1 wherein said halide salt oxide, and the like. The ceramic bodies of the present 1D.- is LiF vention may also be cold pressed and then sintered at about 1000 C. for 1-2. hours under an air, vacuum, or hydrogen environment and then annealed (as in the hot pressing operation). The best results occur when the cold pressed bodies are sintered under reducing conditions,

5 6 5. The ceramic body of claim 1 wherein said halide salt 3,462,378 8/ 1969 Remelka 252-629 is NaF, 3,586,642 6/1971 Matsuo et a1 106-39 R References Cited UNITED STATES PATENTS OSCAR R. VERTIZ, Pnmary Exammer 5 I. COOPER, A t t E 3,000,745 9/1961 Cianchi 10s 39 R 8518 an Xammer 2,695,239 11/1954 Oshry 106-39 R US. Cl. X.R. 3,340,074 9/1967 Herczog 10639 R 10639 R 

